Resistor element for electric-current controllers and process of making the same



L. BRADLEY.

-RESISTOR ELEMENT FOB ELECTRIC CURRENT CONTROLLERS AND PROCESS OF MAKING THE SAME.

APPLICATION FILED DEC. I6I I9I8.

Patented Mar. 8, 1921.

2 SHEETS-SHEET I.

I-rweviiiofl L rcciefiradzq g 'III L. BRADLEY. RESISTOR ELEMENT FOR ELECTRIC CURRENT CONTROLLERS AND PROCESS OF MAKING THE SAME. APPLICATION FILED DEC-M11918.

1,370,728, PatentedMar.8,-1921.

2 SHEETS-SHEET 2.

rib-196112707 I subject to the flow of currents LYNDE BRADLEY, OF MILWAUKEE, WISCONSIN.

RESISTOR ELEMENT FOR ELECTRIC-CURRENT CONTROLLERS AND PROCESS OF MAKING THE SAME.

Specification of Letters Patent.

Patented Mar. 8, 1921.

Original application filed. July 29, 1916,- serial No. 112,009. Divided and this application Med December T 0 all whom it may concern:

Be it known that I, LYNDE BRADLEY, a citizen of the United States, residing at Milwaukee, in the county of Milwaukee and State of Wisconsin, have invented certain new and useful Improvements in Resistor Elements for Electric-Current Controllers and Processes of Making the Same, of which the following is a specification.

My invention relates to improvements in electric current controlling devices, with especial reference to that class of controlling devices disclosed in my patent application Serial No. 112,009, filed July 29, 1916, of which this is a divisional application. The general object of such devices is to vary the protecting resistance in circuit with the motor or other electrically operated apparatus and to commutate the circuits connected therewith.

The object of the invention hereinfully explained is to provide a resistance material of carbon in either its amorphous or crystalline form and a method of making the same whereby it can be made more cheaply and easily than heretofore, in a form which is homogeneous, free from voids, and in which the original molded faces may be formed during the processes so that no grinding or subsequent shaping is necessary. The resultant product also is more pressure, shock, and heat resisting and therefore less disintegration from use than products heretofore made except by very long, delicate, and expensive processes.

It is a further object of the invention to provide a carbonaceous resistance material possessing relatively high contact resistance and that will remain constant through long continued use and will not have its resistance at normal temperatures permanently altered by excessive heating or pressure or both.

Carbonresistors intended for controlling for use with motors or "other electrical machines or apparatus have heretofore been made by mixing or pugglng finely powdered petroleum coke, gas retort carbon, lamp black or graphite with some liquid carbonizable binder such as caramel, molasses or coal tar; This mixture was then molded under pressure into the shapes desired or squirted into the-form of rods, tubes, or plates by means of a suitable press.

1918. Serial No. 267,079.

The molded or squirted pieces were then baked dry and then fired at a high temperature for the purpose of carbonizing the binder. g

Carbon made in the manner described when used as a resistance element in connection with electric current controllers of the class previously referred to, or when used as a resistance element in electric heating devices, has proved unsatisfactory, due to its property of disintegration or returning to powdered form when repeatedly heated and cooled. I am not certain as to the exact cause of this phenomenon though I attribute it to a multiplicity of minute voids that are necessarily formed and exist between the discrete particles of the carbon and the binder; to the difference of co-efficient of expansion between the particles of finely ground carbon and the carbon formed by carbonizing the binder, or possibly a partial and unequal crystallization or graphitization resulting in internal stress and cleavage or parting planes and also to the stresses set up between the original carbon particles and the carbonized binder due to difference in amount of shrinkage when carbonized,'which results in finally forcing the binder carbon to release its hold or break .apart from the original carbon powder. In

all of these cases it has been found impossible to eliminate the binder which is a con stant source of trouble as stated herein. There is also a certain amount of ash and other incombustible, non-volatile, non-conductors which are often present in considerable amounts in carbon resistors as heretofore made. This results in cases of over-. heating in the formation of films of non conductors gradually forming at the points of contact of the resistor elements which tend to increase the resistance thereof and eventually cause the resistor elements to disintegrate, dueto the arcing. This deterioration of the resistor element composed of the types of carbon heretofore used results in making the controlling devices with which they are associated of comparatively short life, resulting in their becoming quickly inoperative and under some circumstances dangerous.

It isimpossible to make a solid free from voids by any mixture of solids no matter how finely they may be ground or intimately they may be mixed. A solid can be made free from voids by causing it to be set from a liquid but there are many disadvantages in this method unless the liquid .be held in some specific definite shape prior to its becoming set. I have found that I can do this in a way which will at the same time effectually eliminate all solid matter just at or prior to the process of carbonization, probably by solution under the influence of pressure and heat, thus giving me a resultant homogeneous carbon and one which I can mold in various forms and shapes and handle easily and effectively.

With my improved method of making carbon resistors, the material carbonized is a homogeneous solid set from a liquid and no permanent binder remains, merely a temporary shaping means which is dissolved by the liquid material underthe i11- fiuence of heat, and this I call a fugitive binder in that its existence and use are temporary only and it disappears in the final result. The resulting carbon is of the amorphous variety and is of uniform texture and can be used in that condition or graphitized; it possesses a relatively-high contact resistance which is readily varied at will; itwill stand a large amount of pressure; and it will not deteriorate due to variations in temperature below its ignition point or below what I call its critical point being about 400 C. at which point ordinary carbon commences to oxidize rapidly.

I am aware that considerable work has been. done in the making of carbon for use in incandescent lamps, telephone transmitters, and microphones, but the problems to be solved in those arts are quite difi'erent from those in the art of electric current controllers that are called upon to handle heavy currents, stand intense and varying degrees of heat, together with the disturbing effects of great pressure and shock, and in addition the oxidation consequent upon exposure to the atmosphere when in a heated condition. Because of these and other differences the results arrived at in those fields have not produced a carbon resistor element that will operate successfully in an electric current controller of the class described. It isvery important in a resistor element that the completed resistor element beentirely homogeneous, free from voids and also from ash or other non-volatile, non-conducting residue, as their presence results in an element which will gradually break down when subjected to varying degrees of pressure, heat, and air, and the carbon resistor disintegrates or powders up and becomes useless. p

I will now describe one specific example of the manufacture of articles of this form of carbon. I take a sheet or mass of cellulose fibers in the form of pure blotting.

or filter paper and impregnate the material with a synthetic resin formed by reacting phenol with formaldehyde. It is important that the paper be extremely pure and free from ash or other foreign matter. I find that the best results are obtained with a high grade of filter paper, which is almost pure cellulose and which gives me a very perfect fugitive binder. The impregnation is accomplished by dipping the sheets into a container of the resinous liquid, and then hanging them by suitable clamps upon racks in such a manner that the superfluous material is drained away. The sheets are allowed to hang until the reactions have continued sufficiently so that thesheets .are not sticky and can be readily handled. The racks containing them are then placed in an oven and the temperature gradually raised from that of the room to 250 Fahrenheit, over a period of about two hours.

The sheets are then in proper condition to be formed into the shape of the finished carbon articles.- In the specific example described, the sheets are punched into the form of disks by means of a die operated in a punch press. The disks are punched from the impregnated sheets with a diameter of about 25 per cent. greater than is desired when in the form of carbon. After 95 being punched, the disks are stacked alternately with smooth flat steel disks, in a slightly inclined chute, forming columns. These columns are of convenient length and in the specific instance described are 12 inches. These columns are then slipped vertically into tubular retorts which are held bottom side up over the chutes when receiving the disks. The retorts are then supported "vertically in iron racks and into the top of each of the retorts is placed a fairly close fitting cylindrical weight which performs the double function of substantially sealing the retort against the admission of air and preventing the disks from warping during the carbonizing.

The process of carbonizing consists in placing the retorts with their contents in a vertical position in a furnace and raising the temperature sufficiently so that the resin- 1 ous material reacts upon the cellulose forming a homogeneous mass and heating is continued long enough to drive off the volatile matter from this cellulose resinous compound, leaving only the carbon and non- 1 volatile matter. The manufacture of the disk can be completed in this preliminary firing if desired, but a more refractory and lower resistance carbon is produced if the disks are subjected to the more intense heat 12 of the electric furnace, which results in the polymerization and purification of the carbon. Or if desired the carbon may be converted into its graphitic form by introducing into the electric furnace some catalytic .pack or column form of a disk.

Fig. 3 is a sectional view of an assembled resistance unit of the form in which a column or pack of homogeneous amorphous carbon disks or plates are employed as the resistance elements. The efiective resist-' ance is the resistance of contact at the joints between each of the plates comprising the and is varied by increasing or decreasing the pressure on the column, thus bringing the numerous joints into more or less intimate contact. The particular mechanism for compressing the resistor pack forms no Part of the present invention, a suitable mechanism for the purpose being shown, for instance, in Patent No. 823,697, dated May 29, 1916, for electric current controller. A method of and a means for exerting and relieving the pressure on .a column of patent. Since my present invention relates to the variable resistance element rather than to any special means of varying the pressure thereon, it is obvious that the assembled variable resistance unit embodying my present invention may be made in any form and may be used with any type of compressing means. In Fig. 3 a stack of resistor disks or plates 1, as indicated in Fig. 2, are shown within a container 2, which is a metal tube suitably lined with heatresisting electrical insulating material 3, having a fixed contact 4 and a slidable contact 5.

Fig. is a part of a modified form of the resistor element as described, parts being broken away, and is arranged to be supported by a rod passing through the center.

Fig. 5 shows a sheet of pure blotting paper 6, which is suspended by the clamps 7 ready for dipping into the liquid resin which is placed in the container '8.

Fig. 6 shows the disks formed from the dipped, impregnated blotter 6, containedin the retort 10 ready for firing in the gas furnace with the flat steel. disks 11 alternately disposed between the disks 9 and the weight 12 placed in the top of the retort.

Fig. 7 shows the retorts 10 held upright in the gas furnace 13, wherein the charge of disks .in the form of' disks is clearly indicated in that disks therein are carbonized, as hereinbefore stated.

The subject-matter of this invention is not necessarily limited to use in controllers of the class described although that is the most exactin use and is the specific purpose for which use it; also the product and process may be varied for different purposes within the scope of the appended claims.

Having fully described my invention, 1 claim:

1. The process of making a resistor element capable of use in an electric current controller that carries heavy currents, which consists in impregnating a solid binder with a fluid of high carbon content and heating the product so formed so as to eliminate the binder as such and form a homogeneous carbon.

2. A process of making a resistor element capable of use in an electric current con-- troller for handling heavy currents which consists in absorbing a synthetic resin in a substantially pure cellulose paper, solidifying the resin, forming the impregnated paper into disks and then carbonizing the disks sov formed.

3. A process of making a resistor element capable of use in an electric current controller for handling heavy currents. which consists in absorbing a fluid phenol aldehyde resin in a substantially pure cellulose paper, hardening the impregnated mass, forming disks therefrom, and then carbonizing the disks under pressure.

1. A process of making resistor elements for use in electric current controllers capable of handling heavy currents, which consists in impregnating pure cellulose paper with a synthetic resin, hardening the impregnated mass, heating the impregnated mass under pressure so as to cause the resin to dissolve the cellulose therein and then carbonizing the same under pressure.

5. A process of making resistor elements for use in electric current controllers capable of handling heavy currents, which consists in impregnating pure cellulose aper with a synthetic resin, hardening the impregnated mass, heating the impregnated mass under pressure so as to liquefy the resin and dissolve the cellulose therein, carbonizing the same under pressure and then graphitizing the product so formed.

6; A resistor carbon capable of handling heavy currents in an electric current controller which consists of homogeneous carbon formed by carbonizing a solid of high carbon content that has been set' from a liogiid containing a fugitive binder.

A resistor carbon capable of handling heavy currents in an electric current controllerwhich consists of carbon formed by carbonizing a synthetic resin containing a fugitive binder.

8. A heavy resistor carbon capable of handling currents in an electric current controller which consists in homogeneous amorphous carbon'forined by carbonlzing under 5 pressure a phenol cellulose in solution therein. 9. A resistor element for trio current controller consisting of polymerizecl carbon formed. by

aldehyde resin containing use in an elecsetting from a liquid of a synthetic resin containing a fugi- 10 tive binder in solution therein, being substantially free from voids and having the original molded face.

LYN DE BRADLEY. 

